Project Summary/Abstract The opioid overdose epidemic is a public health emergency, claiming more lives each year than HIV/AIDS, motor vehicle accidents, or firearms at their peak. A significant development in illicit drug markets has been the emergence of illicit fentanyl and its high-potency analogues, often manufactured in clandestine labs overseas. Within just the last few years, fentanyl has become a driver of opioid overdose deaths. The escalation in mortality has been rapid and unprecedented, and fentanyl is now implicated in more deaths than heroin or prescription opioids. Fentanyl?s extreme potency leaves very little room for error in discerning between lethal and non-lethal doses. Illicit fentanyl is often used as an adulterant in the heroin supply, and is increasingly sold on the street by name. Recent evidence suggests that demand-side responses to fentanyl have ranged from avoidance strategies to the development of distinct preferences for fentanyl. The wide availability of fentanyl has led to the emergence of a new group of individuals with opioid use disorder (OUD) for whom fentanyl has become a preferred drug of choice. Very little is known about this high-risk group of people who prefer fentanyl (PPF). It is not known how PPF differ from other individuals with OUD who prefer less potent opioids, how they fare in OUD treatment, or how they perceive overdose prevention and treatment interventions. This study seeks to learn more about intentional vs. unintentional fentanyl exposure and preferences among people entering OUD treatment. The study will: (1) Characterize fentanyl preferences among patients starting OUD treatment, (2) Compare 6-month OUD treatment outcomes for PPF and non-PPF, and (3) examine PPF perspectives on public health interventions to prevent overdose and improve OUD treatment engagement. The study will recruit 250 fentanyl-exposed adults newly admitted to a large community OUD treatment program. Participants will complete a structured assessment, and 6-month outcomes will be tracked via innovate data linkage across OUD treatment, healthcare, and judiciary record systems. A subset of 40 participants will complete semi-structured, qualitative interviews focused on fentanyl preferences and perspectives on interventions. This study will yield timely data on the public health emergency of fentanyl, and will inform intervention design for a high-risk group of patients who have developed distinct preferences for fentanyl.